A fuel cell is a device which generates electric energy from hydrogen (a fuel) and oxygen (an oxidizing agent), and it can obtain a high power generation efficiency. A main characteristic of the fuel cell is that it carries out direct power generation without any process of thermal energy or kinetic energy as in a conventional power generation system, and hence the high power generation efficiency can be expected even with a small scale. Another characteristic of the fuel cell is that it discharges less amounts of nitrogen compounds and the like, and generates less noise or less vibration, which means excellent environmental properties. As understood from the above, the fuel cell can effectively use chemical energy of the fuel and has eco-friendly properties. Therefore, the fuel cell is expected as an energy supply system for the 21st century, and much attention is attracted as a new power generation system which is promising in the future and which is usable in various applications for cosmos, vehicles and portable devices in a range from large-scaled power generation to small-scaled power generation. Accordingly, the technical development of the fuel cell has been promoted for practical use.
Above all, a solid polymer fuel cell has characteristics such as a low operating temperature and a high output density as compared with another type of fuel cell. Especially, in recent years, as one configuration of the solid polymer fuel cell, a direct methanol fuel cell (DMFC) has received attentions. The DMFC directly supplies an aqueous methanol solution as the fuel to an anode without changing properties of the solution, and obtains the power by an electrochemical reaction between the aqueous methanol solution and oxygen. Owing to this electrochemical reaction, carbon dioxide is discharged as a reaction product from the anode, and generated water is discharged as a reaction product from a cathode.
Since the aqueous methanol solution has high energy per unit volume as compared with hydrogen, is suitable for storage and has little danger of explosion or the like, the solution is expected to be used in a power source for a car, portable devices (a cellular phone, a laptop PC, a PDA, an MP3 player, a digital camera, a game unit and electronic dictionary (book)) and the like (e.g., see Japanese Patent Application Laid-Open Nos. 2006-040787 and 2006-054075).
Here, in the power generation system constituted using such a fuel cell, a system is adopted in which a chargeable/dischargeable secondary battery is once charged with the power generated by the fuel cell to store the power, and the power is taken from the secondary battery and supplied to a device, so that the power can steadily be supplied to the connected device.
In this case, it is preferable for a user of the device that the secondary battery is quickly charged or constantly fully charged. On the other hand, if the secondary battery is repeatedly charged or discharged in the vicinity of a remaining amount corresponding to 100% (a charge ratio), the cell might be deteriorated. Moreover, it is generally said that, as the cell is nearly fully charged, a storage state deteriorates (see “Cell Application Handbook” edited by Transistor Technology Edition Department, CQ Publishing Co., Ltd. (ISBN4-7898-3446-8). Avoidance of this problem is preferably taken into consideration. When the fuel cell is operated at a small current, the efficiency of the power generation improves. However, much time is required for charging the secondary battery at the small current.
The present invention has been developed to solve such conventional technical problem, and an object thereof is to provide a power source system in which life is lengthened and a highly efficient operation can be realized while meeting user's needs.